This invention relates to the control of centrifugal compressors to prevent surging thereof.
If the volume of gas delivered by a centrifugal compressor falls below a predetermined limit, the compressor surges. For example, if the compressor is arranged to deliver a constant volume of air to a blast furnace, and the varying conditions in the blast furnace causes an increase in the resistance to the flow of the air through the compressor, the compressor will be required to deliver to the blast furnace a greater mass flow of air in order to maintain the said volume of air constant at the higher discharge pressure from the compressor. If, however, sufficient air is not available at the compressor inlet, the compressor will run out of air with the result that there will be a reverse flow of air through the compressor, i.e. a surge cycle will occur. If the resistance to the flow of air through the compressor is not then reduced, the surge cycle will be repeated until the correct volume of air flows through the compressor.
Such surging is highly undesirable since the resultant vibration, noise and overheating can lead to mechanical damage and ultimate wrecking of the compressor and of associated instrumentation and ducting connected thereto.
The compressor must therefore be controlled to prevent surging under all operating conditions, and this is normally achieved either by re-circulating, when necessary, a flow of the gas which has been compressed in the compressor from the outlet to the inlet thereof through a by-pass duct, or by blowing off some of the gas discharged from the compressor.
Precise surge control is desirable to increase the operating range of the compressor and to avoid unnecessary energy losses. Such precise surge control should be responsive to changes in the composition, inlet pressure and inlet temperature of the gas entering the compressor and, in many cases, should be such as to ensure that the compressor is operated as closely as possible to the surging condition in order to obtain the best efficiency.
The conventional method of defining the surge point, i.e., the conditions in which the compressor will surge, has consisted in determining the relationship between the outlet pressure of the compressor and the volumetric flow through the compressor inlet. The method is not sufficiently accurate however since it takes no account of variables such as pressure, temperature, molecular weight and supercompressability of the gas entering the compressor. Consequently, when this method is used, the compressor is liable to surge "for no apparent reason".
In an attempt to allow for some of these variables, compressor manufacturers often supply a family of curves defining surge, each such curve showing the said relationship between the outlet pressure and the inlet volumetric flow for predetermined conditions of inlet temperature and pressure. Not only, however, is it difficult in practice to use such a family of curves, but also it is by no means necessarily apparent in practice which particular curve is applicable since the value of a variable such as the said inlet pressure may not be very accurately known and does not necessarily remain constant. Consequently, it is not practicable to operate at all close to the surge point as defined by the respective curve, and this can mean that the compressor is necessarily very inefficiently operated.
Various attempts have therefore been made to control a centrifugal compressor otherwise than by merely determining the relationship between the outlet pressure of the compressor and the inlet volume thereof. For example, in British patent specification No. 1,209,057 the compressor is controlled in accordance with the formula ##EQU3## where h is the pressure difference across a throttling element in the intake to the compressor, p.sub.1 and p.sub.2 are respectively the inlet and outlet pressures of the compressor, .phi. and .psi. are constants which depend respectively on the particular compressor and throttling element used, and a and b are constants which depend on the value of the compressor ratio p.sub.2 /p.sub.1 and on the polytropic exponent n. This formula, however, is derived mathematically from the proposition that surging in a centrifugal compressor depends only on the angular velocity N of the compressor rotor, whereas in fact it also depends on the temperature T, the supercompressability Z, the ratio of the specific heats .gamma. and the molecular weight M.W. of the inlet gas. Consequently the said formula is applicable only to low values of the compression ratio.